184 Lloyd Williams . — Studies in the Dictyotaceae. 
would be instructive to know whether they hold good for other localities 
also. 
As the sexual cells pass through the various stages of development 
at approximately the same dates, it is far easier to find any required 
mitosis here than in the gametangia of the Fucaceae or in the tetrasporangia 
of the Dictyotaceae. After the preliminary trials one has only to consult 
the tide tables for the particular locality to know within two or three 
days when certain stages may be found. 
Although Thuret and Reinke have given excellent figures of the 
sexual cells of Dictyota no description of their cytology has as yet been 
published. 
I. The Development of the Oogonia. 
The oogonia instead of being isolated, as is the case with the tetra- 
sporangia, are grouped together in sori, each sorus consisting of twenty-five 
to fifty oogonia, closely packed together without paraphyses or any other 
accessory cells. In the case of weak plants, or under adverse conditions, the 
number may be reduced to half a dozen or fewer. The sori are scattered all 
over the two surfaces except the basal portion, the apices, and a very narrow 
band at the margins of the thallus. The sori of the new crop appear 
between the scars of the older ones, and, in the case of an elongating plant, 
in acropetal succession on some of the surface cells which have sufficiently 
matured since the preceding crop. This process may go on till the whole 
of the available surface has been used up, then the plant dies. In no 
case are gametangia produced by the stalk-cells left by preceding sori. 
As in the case of the tetrasporangium the rudiment of the oogonium 
is one of the small cells of the assimilating layer, which increases in 
dimensions till the free part is about three or four times the height of the 
original cell (PI. XII, Fig. 3). The nucleus then divides, and a stalk-cell 
is cut off. The oogonium mother-cell increases somewhat in size, but 
it undergoes no further division, the nucleus becoming that of the ovum 
without an additional mitosis. This is essentially different from the state 
of affairs in the oogonium of the Fucaceae, where eight nuclei are produced 
even in the cases where only one egg ultimately matures. In the Fucaceae 
also, where there is no asexual generation, reduction takes place in the 
first of the three oogonial mitoses ; in Dictyota , as shown in the preceding 
paper, it is brought about in the first of the two tetrasporic divisions. In 
a few days after the separation of the stalk-cell the apex of the oogonium 
ruptures and the naked oosphere is liberated into the water, where 
fertilization takes place. 
The stalk-cell mitosis is accomplished from one to four days before 
the highest spring tide, the variation in the time being dependent on the 
length of the interval between two spring tides, the conditions with respect 
